Development of high-brightness LEDs, and their incorporation into Lamps designed to replace incandescent bulbs has revolutionized the lighting industry in recent years. One of the advantages of an LED lamp over an incandescent lamp is its greater efficiency in converting electric energy into light. This measure of how well a light source produces visible light is known as luminous efficacy. A typical incandescent bulb produces about 14-17.5 lumens per watt, and most halogen lamps produce about 16-24 lumens per watt. In comparison, LEDs achieving 80-150 lumens per watt are now common. Even when considering the power that is lost in the driving circuitry of an LED lamp which may be 60-80% efficient, LED lamps that are three to six times as efficient as incandescent and halogen bulbs are easily achievable. Thus an LED lamp designed to replace a halogen bulb lamp would draw much less power from the AC mains. In installations employing many such light fixtures, a great savings in electric energy can be realized by replacing the halogen lamps with comparable LED Lamps.
Lamps employing fluorescent bulbs are common due to their increased efficiency over incandescent, however fluorescent bulbs have other undesirable characteristics. Fluorescent light tubes contain a small quantity of mercury that can be harmful to the environment and to human health. In order to properly dispose of the fluorescent lights special care must be taken and special hazardous waste landfills must be used. Thus, proper disposal of fluorescent lamps is expensive, and the more common improper disposal is damaging to the environment. For these and other reasons, LED based lamps are becoming mainstream in the lighting industry.
As LED lighting manufacturers have sought to get the most luminous intensity from their products in order to replace incandescent and fluorescent lights, they have naturally sought the brightest LEDs. LED manufacturers continually strive to get the most luminous efficacy from their LEDs. Common in the industry are 1 Watt LEDs which are now approaching 150 lumens. These 1 Watt LEDs are commonly seen in linear lighting products, spaced an inch or more apart in a linear row. Because of the heat generated by the LEDs, which must be dissipated away from the LEDs in order to prevent damage to the die and in order to get the most life out of the product, these 1 Watt LEDs can't practically be spaced any closer to each other. All LED lighting designers face the same common challenges of heat dissipation, luminous intensity, price point efficiency, product reliability, etc. Thus, in terms of linear LED lighting products, 1 Watt LEDs are commonly used and spaced as described. Examples of these existing prior art linear Products include HPNLS and HPNFC from Boca Flasher, Inc., Lumentask Undercabinet Lighting from GM Lighting LLC, and LED Undercabinet and Linear Lighting products from Juno Lighting LLC. FIG. 1 shows a common prior-art linear LED task light with 1 Watt LEDs (102) mounted to a PCB (104) contained in a thermally conductive housing (106).
While these linear LED lighting products all have the advantages over incandescent and fluorescent lights that were discussed above, they suffer from one disadvantage which becomes amplified when the products are used to illuminate an area or surface that is in close proximity to the lamp. This disadvantage is in the uneven lighting pattern that results from the nature of the LEDs themselves.
Unlike incandescent and fluorescent bulbs, LEDs are directional light sources. That is, LEDs emit light with an intensity that is greatest when viewed on-axis, and drops off as you move off axis. LEDs are lambertian emitters, which obey Lambert's cosine law. Lambert's cosine law says that the radiant intensity or luminous intensity observed from an ideal diffuse radiator is directly proportional to the cosine of the angle θ between the observer's line of sight and the surface normal. The law is also known as the cosine emission law or Lambert's emission law. FIG. 2 shows a graph of this lambertian radiation pattern.
In practice, LED manufacturers typically add a small lens to the surface of the LED to modify this emission pattern by widening or narrowing the intensity profile. The intensity pattern of the LEDs is often specified by the manufacturers as a beam angle. The beam angle is the angle between those points on opposite sides of the beam axis where the intensity drops to 50% of maximum. It can be seen from FIG. 2 that the beam angle of a simple lambertian emitter is 120°. Even when widened or narrowed by the manufacturer's lens, the light pattern is always greatest directly on-axis with the LED.
LED lighting manufacturers must deal with this LED optical property when designing LED lighting products. Depending on the desired characteristics of the light, various methods of focusing or diffusing the light with secondary lenses, reflectors, directional optics, multiple mounting planes, diffusing covers, etc. are sometimes employed. However, these secondary optics add to the cost of the lighting product. When possible, a simple opaque or frosted cover on the LED lamp is used, and is often acceptable when the illuminated surface is sufficiently distant from the lamp so that the pattern of light from each LED can overlap and blend.
The emission pattern of light from these prior-art linear LED lights becomes problematic when the light is used to illuminate a close proximity surface such as with under cabinet lighting or task lighting in modular office furniture. In these and other task light applications, the lights are typically within 18 inches of the surface that they are illuminating.
FIGS. 3 and 4 illustrate this problem. FIG. 3 shows the non-uniform illumination pattern resulting from a surface (320) that is relatively close to a multiple point-source light (300). In this case, the LED emitters (310) each produce a circle of higher intensity illumination which may overlap but, do to the short distance to the work surface (320), do not sufficiently “blend” into an even illumination pattern (330).
FIG. 4 shows another undesirable effect of close-proximity, multiple point-source lighting such as is common in prior-art products. Referring to FIG. 4, each LED (410) can be seen as a point-source of light illuminating a close object (420). the LEDs (410) each illuminate the object (420) from a different angle relative to the work surface (430), and therefore each produce a unique shadow (440) on the work surface (430). The result is a multiple “stepped” shadow (440) of the object (420) which emanates out in linear fashion parallel to the linear light (400). This multiple point-source shadowing can be dramatic and unpleasing.
Because of the above described deficiencies with prior art LED lamps, there exists a need in the industry for a linear LED task light capable of evenly illuminating close proximity work surfaces while minimizing point-source shadowing, without requiring expensive secondary optics or reflectors to achieve the same.
It is an object of the present invention to provide a complete linear LED task light which satisfies energy code requirements of a maximum 7½ Watts per foot under normal operation, capable of evenly illuminating a close proximity work surface, and which minimizes the effects of multiple point-source shadowing seen with prior-art lights. It is a further object of the present invention to comply with all LEED certification requirements, and to be California Title 24 compliant. It is a further object of the present invention to provide a selectable higher capacity illumination mode using greater than 7½ Watts per foot, with an integral timer which reduces the output after a preset interval. It is a further object of the present invention to save power by automatically shutting off when no one is occupying the work area.